Measurement of fatigue of a metal material uses fatigue test piece 10 in which fatigue crack 12 is formed, as shown in FIG. 2. The shape and size of piece 10 are defined in, e.g., B. S. 5762-1979 (Methods for crack opening displacement testing). Test piece 10 is defined such that crack 12 having a predetermined length is formed from the bottom portion of notch 11 formed in the central portion of piece 10. During formation of crack 12, a load must be applied on test piece 10 while observing the length, i.e., the distal end position of crack 12.
Various conventional methods for detecting the distal end position of crack 12 have been used. Among these methods, noncontact detection methods are preferable. The noncontact detection methods include a method for optically monitoring crack 12 and a method for detecting a magnetic change caused by crack 12 as a change in eddy current, if piece 10 is a magnetic substance. However, in the method using the eddy current, the detection result varies in accordance with the magnetic characteristics of the piece, and a crack position cannot be precisely detected. Therefore, demand has arisen for an apparatus which can precisely and easily detect a crack position by an optical method.